Howard A. Johnsen

Ion‐assisted pulsed laser deposition of cubic boron nitride films

Ion‐assisted pulsed laser deposition has been used to produce films containing ≳85% sp3‐bonded cubic boron nitride (c‐BN). By ablating from a target of hexagonal boron nitride (h‐BN), BN films have been deposited on heated (50–800 °C) Si(100) surfaces. The growing films are irradiated with ions from a broad beam ion source operated with Ar and N2 source gasses. Successful c‐BN synthesis has been confirmed by Fourier transform infrared (FTIR) spectroscopy, high‐resolution transmission electron microscopy (TEM), selected‐area electron diffraction, electron energy‐loss spectroscopy, and x‐ray diffraction. The films are polycrystalline and show grain sizes up to 300 A. In addition, Rutherford backscattering, elastic recoil detection, and Auger electron spectroscopies have been used to further characterize the samples. The effects of varying ion current density, substrate growth temperature, growth time, and ion energy have been investigated. It is found that stoichiometric films with a high c‐BN percentage ca...

The Multiplexed Chemical Kinetic Photoionization Mass Spectrometer: A New Approach To Isomer-resolved Chemical Kinetics

We have developed a multiplexed time- and photon-energy-resolved photoionization mass spectrometer for the study of the kinetics and isomeric product branching of gas phase, neutral chemical reactions. The instrument utilizes a side-sampled flow tube reactor, continuously tunable synchrotron radiation for photoionization, a multimass double-focusing mass spectrometer with 100% duty cycle, and a time- and position-sensitive detector for single ion counting. This approach enables multiplexed, universal detection of molecules with high sensitivity and selectivity. In addition to measurement of rate coefficients as a function of temperature and pressure, different structural isomers can be distinguished based on their photoionization efficiency curves, providing a more detailed probe of reaction mechanisms. The multiplexed three-dimensional data structure (intensity as a function of molecular mass, reaction time, and photoionization energy) provides insights that might not be available in serial acquisition, as well as additional constraints on data interpretation.

Implementation of laser-induced breakdown spectroscopy as a continuous emissions monitor for toxic metals

Laser-induced breakdown spectroscopy (LIBS) has been considered for some time as a potential CEM method for toxic metals. Recently, improvements in sampling methodology and signal processing have allowed LIBS to achieve detection limits below the proposed MACT limits for 5 out of 6 of the RCRA metals. This paper discusses performance improved by nearly 2 orders of magnitude of this in situ monitoring technique following implementation of conditional analysis. Results from trial burns at two incinerators and at a DoD contained burn facility are highlighted. At the incinerators, implementation of conditional analysis yielded much lower detection limits than previously reported using the LIBS technique. At the contained burn facility, reproducible, transient Pb measurements were recorded in real-time for concentration values that varied by more than two orders of magnitude. Method detection limits of between 2 and 100 μg/dscm are reported for toxic metals Be, Cd, Cr, Hg, and Pb.

Temporal Gating for the Optimization of Laser-Induced Breakdown Spectroscopy Detection and Analysis of Toxic Metals

Optimal temporal gating for laser-induced breakdown spectroscopy (LIBS) analysis was investigated for a select group of toxic metals, namely the Resource Conservation and Recovery Act (RCRA) metals arsenic, beryllium, cadmium, chromium, lead, and mercury. The differing rates of decay between the continuum plasma emission and the atomic emission were used as a means to maximize the signal-to-noise ratio of the atomic emission lines for these six metal species. Detection windows were investigated corresponding to delay times from 2 to 50 μs following the plasma-initiating laser pulse. For the current experimental conditions, it is concluded that the relatively short delay time of 12 μs is optimal for the detection of arsenic, beryllium, cadmium, and mercury, while a longer delay time of 50 μs is optimal for the detection of chromium and lead. The reduced atomic emission intensity at relatively long delay times is compensated for by the use of long detector gate widths. Estimated detection limits are reported for the six metal species based on the optimized temporal gating and ensemble averaging of multiple laser pulses, and the implications for simultaneous metals monitoring are discussed.

Substrate effects in cubic boron nitride film formation

We have examined the effect of the substrate in cubic boron nitride (c‐BN) film synthesis by depositing BN films on a variety of materials using ion‐assisted pulsed laser deposition. Using optical modeling, we estimated the c‐BN content of the films from the measured infrared reflectance spectra. We find less c‐BN in films grown on metal substrates than in films grown on Si and SiC/Si substrates. The c‐BN content of the films decreased with decreasing substrate microhardness. This result is qualitatively consistent with the model of stress‐induced c‐BN film formation.

A laser-based technique to continuously monitor metal aerosol emissions

Abstract We are developing an instrument to continuously monitor metal aerosol emissions in applications including industrial process vents (e.g., exhaust stacks from electroplating baths), waste treatment processes (incinerators), and boilers and industrial furnaces (coal-fired power plants). The monitoring technique is based on laser spark spectroscopy (LASS; also known as laser-induced breakdown spectroscopy - LIBS), in which a pulsed laser is used to rapidly heat a particle and produce a plasma (or laser ‘spark’). The light emission from the spark is spectrally resolved and analyzed to identify the elemental constituents of the particle and quantify the abundance of the measured species. One feature of LASS is that it can measure atomic species embedded in either solid particles or fine liquid droplets, which account for a large percentage of metal emissions from applications of interest. In the initial work described here, we have focused on the application of the LASS technique for measuring chromium emissions from electroplating baths. This paper describes the approach used for measuring the total chromium concentration in laboratory simulations of electroplating aerosols. Chromium concentrations less than 1 mg/scm can be measured. This work forms the basis for future applications to incineration and fossil power plants.

Aerosol generation system for development and calibration of laser-induced breakdown spectroscopy instrumentation

An aerosol generation system is described that enables the production of precise mass flow streams of well-characterized, submicron-sized aerosol particles. A pneumatic-type nebulizer is used to nebulize aqueous solutions of dissolved metals, which subsequently dry in a gaseous co-flow, producing a gaseous stream of dispersed, fine solid particles with a known mass concentration. Gravimetric calibration of nebulizer demonstrates the precise nature of the device for aerosol generation. Representative iron-based (spherical) and titanium-based (nonspherical) particles are analyzed using transmission electron microscopy and x-ray diffraction. Both aqueous metals form metallic oxides, namely, FeO and TiO, and the measured particle sizes are in the range 10–100 nm. The measured particle size and composition are used to calculate the size distribution of droplets produced by the nebulizer, which yield a mean droplet diameter of 524 nm. The nebulizer droplet distribution is well described by a log–normal distribution. Using the aerosol generator as a calibration source of aerosol particles for laser-induced breakdown spectroscopy, linear calibration curves were produced for titanium over mass concentrations ranging from 0 to 4425 μg/m3.

Laser-driven hydrothermal process studied with excimer laser pulses

Previously, we discovered [Mariella et al., J. Appl. Phys. 114, 014904 (2013)] that modest-fluence/modest-intensity 351-nm laser pulses, with insufficient fluence/intensity to ablate rock, mineral, or concrete samples via surface vaporization, still removed the surface material from water-submerged target samples with confinement of the removed material, and then dispersed at least some of the removed material into the water as a long-lived suspension of nanoparticles. We called this new process, which appears to include the generation of larger colorless particles, “laser-driven hydrothermal processing” (LDHP) [Mariella et al., J. Appl. Phys. 114, 014904 (2013)]. We, now, report that we have studied this process using 248-nm and 193-nm laser light on submerged concrete, quartzite, and obsidian, and, even though light at these wavelengths is more strongly absorbed than at 351 nm, we found that the overall efficiency of LDHP, in terms of the mass of the target removed per Joule of laser-pulse energy, is lo...

Detection of Surface Contaminant Residue by Tunable Infrared Laser Imaging

We report the development of a new, real-time non-contacting monitor for cleaning verification based on tunable infrared-laser methods. New analytical capabilities are required to maximize the efficiency of cleaning operations at a variety of federal (Department of Defense [DoD] and Department of Energy [DOE]) and industrial facilities. These methods will lead to a reduction in the generation of waste streams while improving the quality of subsequent processes and the long-term reliability of manufactured, repaired or refurbished parts. We have demonstrated the feasibility of tunable infrared-laser imaging for the detection of contaminant residues common to DoD and DOE components. The approach relies on the technique of infrared reflection spectroscopy for the detection of residues. An optical interface for the laser-imaging method was constructed, and a series of test surfaces was prepared with known amounts of contaminants. Independent calibration of the laser reflectance images was performed with Fourier transform infrared (FTIR) spectroscopy. The performance of both optical techniques was evaluated as a function of several variables, including the amount of contaminant, surface roughness of the panel, and the presence of possible interfering species (such as water). Finally, detection limits for generic hydrocarbon contaminants were evaluated as a function of system noise level.

Development of a probe for in-situ measurements of major species in a pool fire using multiplexed near-IR TDLAS

In this paper we discuss the development of a tunable diode laser absorption spectroscopy probe to simultaneously measure, in situ concentrations of 4 major species (CO, C2H2, CH4, and H2O) and the gas phase temperature in a pool fire. The difficulty in making these types of measurements is intrinsic to the environment itself. A large fire is composed of very hot (> 1000 K), turbulent gases and highly radiating and absorbing soot particles. Fiber optic cables are used to transport laser radiation into the fire via a water-cooled probe. This paper focuses on probe design issues, such as the optimization of open path, multipass optics for a turbulent, particulate- laden flow, and the application of high-frequency wavelength modulation spectroscopy to frequency-domain multiplexing of diode lasers.